A monitoring device for an elevator is known from WO 02/12109 A1. The known monitoring device comprises a number of contactlessly actuable switching devices which are serially connected together to form a safety circuit. The switching devices have to have a specific state in order to be able to safely perform an intended action. In particular, in normal operation it has to be ensured in the case of an elevator system that all doors remain closed and mechanically locked when an elevator car of the elevator system moves. If the elevator car does not move, then a shaft door may be opened only if the elevator car is present at this shaft door. In the case of the known monitoring device each switching devices comprises an active unit and a passive unit, wherein the active unit and the passive unit are so constructed that the passive unit is excited exclusively by a pattern generated by the active unit. The active unit is in this regard constructed as an interrogation unit. The passive unit is constructed as a response unit. The interrogation unit is in that case so constructed that it is in a position of transferring data to the response unit and receiving data from the response unit. A first coil of the interrogation unit and a second coil of the response unit are constructed as antennae. The interrogation unit transmits energy to the response unit by way of an electromagnetic field. This is carried out by an electromagnetic coupling, since the energy transmission functions similarly to a transformer where the energy is transferred from the primary winding to the secondary winding through a narrow coupling. The energy coupled in by way of the electromagnetic field is temporarily stored by the response unit in an energy store. As soon as the response unit has received sufficient energy it is functionally capable and responds in a very specific mode and manner to the pattern generated by the interrogation unit.
The monitoring unit known from WO 02/012109 A1 has the disadvantage that the interrogation unit has to be permanently supplied with electrical energy by an energy supply, for example a supply mains. If, for example, this energy supply occasionally fails or an energy transmission to the interrogation unit for the monitoring unit is disturbed then the monitoring device is no longer functionally capable, since due to the absence of energy a charging of the energy store of the passive unit then cannot take place by way of the active unit. It thus can no longer be recognized whether a door is open. If the door is closed again during the power failure, such a possible action remains unrecognized.